[COLLECTIONS-654] Add class SortedProperties to sort keys.

src/changes/changes.xml

@@ -42,6 +42,9 @@
     <action issue="COLLECTIONS-586" dev="ggregory" type="add" due-to="Shailender Bathula, Gary Gregory">
       PatriciaTrie prefixMap clear throws NullPointerException. 
     </action>
+    <action issue="COLLECTIONS-654" dev="ggregory" type="add">
+       Add class SortedProperties to sort keys.
+    </action>
   </release>
   <release version="4.1" date="2015-11-28" description="This is a security and minor release.">
     <action issue="COLLECTIONS-508" dev="tn" type="add">

src/main/java/org/apache/commons/collections4/properties/SortedProperties.java

@@ -0,0 +1,51 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.commons.collections4.properties;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.Enumeration;
+import java.util.List;
+import java.util.Properties;
+import java.util.Set;
+
+import org.apache.commons.collections4.iterators.IteratorEnumeration;
+
+/**
+ * A drop-in replacement for {@link Properties} for sorting keys.
+ * <p>
+ * Overrides {@link Properties#keys()} to sort keys. Allows other methods on the superclass to work with sorted keys.
+ * </p>
+ * 
+ * @since 4.2
+ */
+public class SortedProperties extends Properties {
+
+    private static final long serialVersionUID = 1L;
+
+    @Override
+    public synchronized Enumeration<Object> keys() {
+        final Set<Object> keySet = keySet();
+        final List<String> keys = new ArrayList<String>(keySet.size());
+        for (final Object key : keySet) {
+            keys.add(key.toString());
+        }
+        Collections.sort(keys);
+        return new IteratorEnumeration<Object>(keys.iterator());
+    }
+}

src/test/java/org/apache/commons/collections4/properties/SortedPropertiesTest.java

@@ -0,0 +1,38 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.commons.collections4.properties;
+
+import java.util.Enumeration;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class SortedPropertiesTest {
+
+    @Test
+    public void testKeys() {
+        final SortedProperties sortedProperties = new SortedProperties();
+        for (char ch = 'Z'; ch >= 'A'; ch--) {
+            sortedProperties.put(String.valueOf(ch), "Value" + ch);
+        }
+        final Enumeration<Object> keys = sortedProperties.keys();
+        for (char ch = 'A'; ch <= 'Z'; ch++) {
+            Assert.assertEquals(String.valueOf(ch), keys.nextElement());
+        }
+    }
+}